1. Field of the Invention
The invention relates to a data transmission system and a method, in particular for frame-oriented digital data transmission of a plurality of useful signals embedded in a carrier signal, using time-division multiplex operation, rate matching being undertaken between at least one useful signal and the carrier signal by means of stuff locations.
2. Description of the Prior Art
The importance of digital data transmission for the information society of the 21st Century is increasing strongly. Important applications for this are, for example, voice and data transmission in telecommunication networks and via the Internet. A major part of digital data transmission is presently accomplished by means of optical signals via optically conducting fibres, since transmission via an optical channel (OCh) generally permits higher data rates than transmission of electric signals via metallic conductors.
Standards are defined by the International Telecommunication Union (ITU) in order to ensure compatibility of the data formats between different users in a network. Frequently used standards are the ITU-T G.813, ITU-T G.958, ITU-T G.825, ITU-T G.707 and ITU-T G.975 Standards, which are hereby rendered in full measure the subject matter of the disclosure of this description. A flexible data format is achieved by virtue of the fact that the data are embedded in frames which typically contain an overhead and useful data sections (payload envelope). The useful data to be transmitted are embedded in the useful data sections. The overhead serves in this case, inter alia, for transmitting control information. Regarding the processing of overheads of optical channels in the case of frame-oriented data transmission via optical channels, reference may also be made to Ballintine, J. E. “Data Format and Generic Processing for OCh-OH” , wave star, volume 420.200.11, issue 1.0, which printed publication is hereby rendered in full measure the subject matter of the disclosure of this description.
In order to transmit a plurality of useful signals (clients) via the same physical channel, the useful signals are typically multiplexed. For example, four useful signals or secondary streams with data rates of respectively 2.488 gigabits per second (Gbit/s) (≈2.5 Gbit/s) are mapped into a carrier signal (server) of approximately 10.7 Gbit/s using time-division multiplex operation. Known data hierarchies typically used for this purpose are the synchronous digital hierarchy (SDH) and the synchronous optical network (SONET).
In these known transmission systems in accordance with SDH or SONET, data are typically transmitted by means of synchronous transport modules (STM). A synchronous transport module contains a dedicated STM section overhead (STM-SOH) and a useful data area in which virtual containers with useful data are embedded.
A central problem in digital data transmission is the handling of different data rates between the useful signals (clients) and the carrier signal. For example, if the rate of the useful signal is higher than the rate of the carrier signal, excess data must be stored as stuff bytes in stuff locations (negative stuff opportunities) in order to achieve rate matching. For this purpose, each synchronous transport module in the STM section overhead contains three stuff bytes for rate matching. In order to locate the start of the virtual container, which start can be displaced by the stuffing, there is, moreover, located in the section overhead of each synchronous transport module a pointer which points as start value to the first byte of the virtual container and is decremented or incremented during stuffing.
In the transmission of the synchronous transport modules in accordance with SDH or SONET, stuff locations are therefore reserved inside each STM section overhead. This method proves, however, to be disadvantageous, since, with reference to the selected example (4×2, 5G→10G), a stuffing area for each of the four useful signals is reserved in each OCh frame and reassigned in the receiver, something which is associated with a high degree of processing complexity and therefore requires a high and cost intensive outlay on hardware and software.
Moreover, in transmission using so-called STM-N/OC-M multiplexers (which operate according to the ITU-T G. 707 Standard), the STM section overhead (STM-SOH) is always truncated. The said hierarchies are therefore data-transparent and time-transparent only with reference to the administrative data units, that is to say with reference to the administrative units, AU-x data units of the synchronous digital hierarchy, and with reference to the equivalent data units of the synchronous optical network. The original timing and the STM section overhead are not transmitted. Consequently, the synchronous digital hierarchy and the synchronous optical network are not completely data-transparent and time-transparent, that is to say bit-for-bit data preservation is not ensured during transmission. As a result, inter alia the outlay on hardware and software is increased, since it may be necessary to generate a new overhead in the receiver.
Moreover, it is desirable also to transmit data signals whose data rate is known although their data structure is unknown. Such data signals are denoted as clear-channel signals. Examples of such clear-channel signals are signals in accordance with the Ethernet standard or the Internet protocol (IP).
When such a clear-channel signal is to be transmitted by means of a transmission system which operates in accordance with the synchronous digital hierarchy, it is firstly translated into the structure of the synchronous digital hierarchy, that is to say the signal is transmitted neither in a data-transparent nor in a time-transparent fashion. Moreover, this translation requires a high degree of processing complexity and thereby necessitates high costs.
One object of the invention is therefore to make available a transmission system and/or a method of the type mentioned at the beginning, which avoid or at least reduce the disadvantages of the prior art.
A further object of the invention is to make available a transmission system and/or a method which make available data-transparent and/or time-transparent transmission, in particular of data signals in accordance with the synchronous digital hierarchy, the synchronous optical network or clear-channel signals.
A further object of the invention is to make available a transmission system and/or a method which cause a lesser degree of complexity and a lower outlay on costs.
A further object of the invention is to make available a transmission system and/or a method which permit reliable data transmission at a low bit error rate.